Coastal flood defence and harbour regeneration scheme

Background

In 2012, Denbighshire County Council began a major regeneration programme for Rhyl, designed to boost tourism, improve connectivity and protect the town from coastal flooding and erosion.

The works formed part of a wider regeneration stream that included:

• Harbour enhancements
• Sustainable transport links (including the Wales Coast Path)
• Public realm improvements
• Coastal and tidal flood defence infrastructure

The scheme comprised multiple phases, delivered across both the harbour area and the Rhyl promenade (West Parade), combining regeneration objectives with long term climate resilience.

The challenge

The project presented a number of complex technical and environmental challenges:

• Delivering coastal flood protection to withstand a 1 in 200 year storm event, with allowance for 50 years of climate change
• Constructing walls partly within the intertidal zone, with works carried out under tidal conditions
• Integrating new defences into a busy public realm and tourism setting
• Replacing an originally proposed stepped revetment that was rejected for funding reasons
• Designing a deep water quay wall exposed to wave action, surges and tidal forces
• Achieving a solution that was structurally robust, cost effective and visually appropriate

The scheme required a durable, engineerable system that could be adapted to very different coastal and harbour conditions.

The solution

After reviewing alternative solutions, Marshalls Redi Rock™ modular walling system was selected for both the promenade flood defences and the harbour quay wall, replacing the originally proposed stepped revetment.

West Parade – Flood Defence & Promenade

• Construction of approximately 470 metres of primary and secondary flood defence walls
• Defences designed to protect against a 1 in 200 year coastal storm, with climate change allowance
• Redi Rock™ modular walls used to deliver a robust, quickly installed solution
• Flood walls successfully tested during the storms and high tides of November / December 2015

Rhyl Harbour – Quay Wall & Public Square

• Construction of a 188 metre long extended quay wall, up to +7.51 m AOD
• Wall heights up to 16 Redi Rock blocks, reinforced using a Miragrid XT geogrid system
• First use of this reinforced earth Redi Rock system in the UK
• Designed in collaboration with Groundsolve Ltd (geotechnical consultants)
• Concrete foundation approximately 0.5 m thick, founded around –0.5 m AOD
• Additional steel sheet piles (3–4 m long) installed below foundations to prevent scour and provide a 120 year design life
• Reinforced concrete in situ coping beam tying the top course of blocks

Temporary sand bunds were used to extend tidal working windows, allowing works to proceed safely despite a mean high water level of 3.9 m AOD.

Thanks to the modular Redi Rock system, up to 50 blocks per tidal cycle were installed, enabling the full quay wall to be completed in approximately 30 weeks.

The outcome

The completed scheme is now a flagship regeneration and flood defence project for North Wales.

• Long term coastal flood protection for Rhyl
• A resilient harbour structure capable of withstanding wave and surge loading
• No observed settlement following completion
• Successful performance during major storm events
• Enhanced public realm, harbour navigation and pedestrian routes
• Regeneration of Rhyl as a tourism and leisure destination

Jamie Turner, Site Agent at Dawnus Construction, commented:

“The Redi Rock™ product is very simple but massively effective. It’s easy to install, looks fantastic, and the service from Marshalls was second to none.”